Hydraulic valve actuator for acturing a gas-exchange valve

ABSTRACT

An hydraulic valve actuator for actuating a gas-exchange valve in a combustion cylinder of an internal combustion engine has an operating piston delimiting two pressure chambers, of which the lower pressure chamber acting in the valve-closing direction is permanently charged with fluid pressure via which an intake and return line can be charged by, or relieved of, fluid pressure. To brake the gas-exchange valve in the final phase of the closing procedure to reduce the set-down speed, the return line of the upper pressure chamber is split between two discharge openings, which are connected to one another and arranged in the housing with axial clearance, the upper discharge opening being coupled to a restrictor and the lower discharge opening being displaceable relative to the operating piston and disposed in its displacement path such that it may be closed thereby at a defined distance, prior to reaching its upper limit position.

FIELD OF THE INVENTION

The present invention is directed to an hydraulic valve actuator foractivating a gas-exchange valve in a combustion cylinder of an internalcombustion engine.

BACKGROUND INFORMATION

German Published Patent Application No. 198 26 047 describes a hydraulicvalve actuator of this type, which is also referred to as an “actuator”.In this actuator, the lower pressure chamber, via which the operatingpiston is displaced in the direction of valve closing, is continuallycharged with pressurized fluid. The upper pressure chamber, providedwith an intake line and a return line, via which a piston displacementin the direction of valve opening is effected, is selectively chargedwith pressurized fluid via the intake, using control valves, such as 2/2solenoid valves, or it is relieved again to approximately ambientpressure via the return line. A regulated pressure-supply devicesupplies the pressurized fluid. Of the control valves, a first controlvalve connects the upper pressure chamber to a relief line discharginginto a fluid reservoir, and a second control valve connects the upperpressure chamber to the pressure-supply device. In the closed state ofthe gas exchange valve, the upper pressure chamber is disconnected fromthe pressure supply device by the closed second control valve andconnected to the relief line via the open first control valve, so thatthe actuating piston is retained in its closed position by the fluidpressure prevailing in the lower pressure chamber. To open the gasexchange valve, the control valves are switched over, so that the upperpressure chamber is cut off from the relief line and connected to thepressure supply device. The gas-exchange valve opens because theeffective area of the operating piston delimiting the upper pressurechamber is larger than the effective area of the operating pistondelimiting the lower pressure chamber, the magnitude of the openingstroke lift being a function of the generation of the electrical controlsignal applied to the second control valve, and the opening speed beinga function of the fluid pressure applied by the pressure-supply device.To close the gas exchange valve, the control valves are switched overagain, thereby connecting the upper pressure chamber, which is blockedoff from the pressure supply device, to the relief line. The fluidpressure prevailing in the lower pressure chamber guides the operatingpiston back into its upper limit position, so that the gas exchangevalve is closed by the operating piston.

Such a device requires rapid closing of the gas exchange valve and, atthe same time, a low impact speed of the valve member of thegas-exchange valve on the valve seat formed in the cylinder head of thecombustion cylinder. For reasons of noise and wear, this speed must notexceed certain limit values.

To this end, the use of a valve brake has been proposed in GermanPublished Patent Application No. 102 01 167.2, which brake is connectedto the valve member of the gas-exchange valve or to the valve actuator.The valve brake, which acts during a residual closing stroke of thevalve member, includes an hydraulic damping member having a fluiddisplacement volume that discharges via a throttle opening. In oneversion, where the damping member is integrated in the valve actuator,the return line of the upper pressure chamber is split between twodischarge orifices, which are connected to one another and arranged inthe housing with axial clearance. A restrictor is assigned to the upperdischarge orifice, and the lower discharge orifice is situated in thedisplacement path of the operating piston in such a way that it may beclosed by the operating piston prior to reaching the upper limitposition. The throttle opening is realized by a pressure-controlledrestrictor whose control pressure is adjusted as a function of theviscosity of the displacement volume with the aid of an electricallycontrolled hydraulic pressure valve and an electronic control devicethat triggers it. This has the advantage that the valve member isdecelerated during the closing stroke before it reaches its closedposition, the braking effect being independent of the temperature andthe resulting viscosity of the fluid volume displaced via the throttleopening. Since the opening cross section of the throttle opening isreduced with increasing temperature and attendant decreasing viscosity,the flow velocity of the displaced fluid volume through the throttleopening is reduced to the same extent, so that the magnitude of thebraking of the operating piston via the damping member remainsapproximately constant.

SUMMARY OF THE INVENTION

The valve actuator according to the present invention for actuating agas-exchange valve in a combustion cylinder of an internal combustionengine has the advantage that during the closing stroke of the operatingpiston, that is, with an operating piston moving into its upper limitposition, the lower discharge orifice is closed by the operating pistonfollowing a certain displacement travel. Thus, the fluid from the upperpressure chamber may only be expelled via the restrictor. This lowersthe displacement velocity of the operating piston, so that thegas-exchange valve connected to the operating piston has a reducedclosing speed and the valve member subsequently sets down on the valveseat with considerably reduced striking speed. Since the lower dischargeopening is situated at a distance from the upper limit position of theoperating piston, the braking operation sets in when the valve member ofthe gas-exchange valve is at a certain distance from the valve seat. Themagnitude of the speed reduction may be influenced by adjusting theopening-cross section of the restrictor. If, however, due tomanufacturing tolerances of the gas-exchange valve or as a result ofdifferent thermal expansions of the valve parts, the lift of the valvemember of the gas-exchange valve has changed slightly by the time itsets down on the valve seat of the gas-exchange valve, the displaceabledesign of the lower discharge opening allows an automatic tolerancecompensation. By a corresponding slight shifting of the lower dischargeopening, the braking, which is triggered by the closing of the lowerdischarge opening via the operating piston, sets in with a closingstroke of the operating piston, adapted to the modified valve-memberlift, in such a way that in all closing operations of the gas-exchangevalve the braking of the valve member always sets in at the same pointrelative to the distance from the valve member. This means that thevalve member is decelerated over a constant, tolerance-independentbraking path until it sets down on the valve seat.

According to an embodiment of the present invention, the displaceabledesign of the lower discharge opening is realized in that the lowerdischarge opening is made up of a radial bore penetrating the housingand a radial bore, communicating therewith, in a compensation piston,which encloses the operating piston and is displaceable relative to theoperating piston. On the one side, the compensation piston, which isdesigned such that it is carried along by the operating piston movinginto the upper limit position, axially delimits the upper pressurechamber together with the operating piston. On the other side, itaxially delimits a blockable compensation chamber via its annular endface, which faces away from the upper pressure chamber.

According to a particular embodiment of the present invention, thecompensation chamber is blocked off over the displacement path of theoperating piston. It is released again for a fluid exchange when theoperating piston, moving into its upper limit position, begins to takethe compensation piston along. In this way, when the lower dischargeopening is closed, the compensation piston is still able to move withincertain limits and adjusts the position of the lower discharge openingwith respect to the closed position of the gas-exchange valve, the lowerdischarge opening determining the onset of the braking operation. As aresult, the braking always sets in when the valve member is at preciselythe same distance in front of the valve seat, regardless of tolerancesor thermal expansions occurring in the gas-exchange valve.

According to an alternative embodiment of the present invention, toensure that an axial displacement of the compensation piston is possibleonce the operating piston has closed the lower discharge opening, thecompensation chamber is connected to a fluid reservoir at least as soonas the compensation piston begins to be taken along by the operatingpiston moving into its upper limit position. The connection between thecompensation chamber and the fluid reservoir may also be permanent;however, the restriction that the connection is established only whenthe compensation piston is taken along has the advantage that itprevents the compensation piston from being taken along prematurely, asa result of friction between the compensation piston and the operatingpiston.

Providing the fluid reservoir has the additional advantage that themovement of operating piston out of its upper limit position, which isaccompanied by the opening of the gas-exchange valve, takes place with arelatively great displacement force. This force is reduced following adisplacement travel determined by the fluid reservoir, namely when nofurther fluid volume is able to be expelled into the reservoir from thecompensation chamber. Reducing the displacement force in the subsequentdisplacement path of the operating piston saves energy, since theactuating force required for the further opening of the gas-exchangevalve following the initial opening of the gas-exchange valve is muchlower than the actuating force that is generated during the initialopening of the gas-exchange valve against the high internal pressure inthe combustion cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal section of a valve actuator connected to agas-exchange valve according to an embodiment of the present invention,showing a maximally opened gas-exchange valve.

FIG. 2 shows a longitudinal section of a valve actuator connected to agas-exchange valve according to an embodiment of the present inventionas in FIG. 1, in this case showing the braking onset of the gas-exchangevalve.

FIG. 3 shows a longitudinal section of a valve actuator connected to agas-exchange valve according to an embodiment of the present inventionas in FIGS. 1 and 2, in this case showing a completely closedgas-exchange valve.

FIG. 4 shows a longitudinal section of an embodiment of a modified valveactuator according to the present invention.

FIG. 5 shows a longitudinal section of a valve actuator connected to agas-exchange valve according to an embodiment of the present invention,showing a maximally opened gas-exchange valve.

FIG. 6 shows a longitudinal section of a valve actuator connected to agas-exchange valve according to an embodiment of the present inventionas in FIG. 5, in this case showing a completely closed gas-exchangevalve.

DETAILED DESCRIPTION

The hydraulic valve actuator schematically shown in longitudinal sectionin FIG. 1 is used to activate a gas-exchange valve 10 in a combustioncylinder of an internal combustion engine. Gas-exchange valve 10 has avalve shaft 11 and a valve member 12 disposed on the far end of valveshaft 11 relative to the valve actuator, valve member 12 cooperatingwith a valve seat 13 formed in the cylinder head of the combustioncylinder. Valve seat 13 encloses a valve opening 14, which is closed ina gas-tight manner when valve member 12 sits on valve seat 13.Gas-exchange valve 10 may be an intake valve or a discharge valve of thecombustion cylinder.

The valve actuator, also called an actuator, for activating gas-exchangevalve 10, which represents a double-acting working cylinder, has ahollow-cylindrical housing 15 and an operating piston 16 guided inhousing 15 so as to be displaceable in an axial direction. Operatingpiston 16 is fixedly connected to valve shaft 11 and, in a displacementlimit position shown in FIG. 3 and referred to as upper limit positionin the following, holds gas-exchange valve 10 closed. In a displacementlimit position, in the following referred to as lower limit position andshown in FIG. 1, it opens gas-exchange valve 10 to the maximum. Byeffective areas having different sizes, operating piston 16 axiallydelimits two volume-variable pressure chambers 17, 18 in housing 15. Theeffective area delimiting the right pressure chamber in FIG. 1, referredto as upper pressure chamber 17 in the following, is larger than theeffective area delimiting the left pressure chamber in FIG. 1, referredto as lower pressure chamber 18 in the following. Lower pressure chamber18 is permanently connected to a pressure-supply device 20, whichdelivers fluid, such as hydraulic oil, that is under high pressure.Pressure-supply device 20 is represented in simplified form by ahigh-pressure pump 19, which draws in fluid from a fluid reservoir 21and provides the fluid, which is raised to high pressure, at output 201of pressure-supply device 20. As a rule, pressure-supply device 20 alsoincludes a reservoir and a non-return valve. Upper pressure chamber 17has an intake 22 and a return line 23, return line 23 being connected toa first control valve 25 by way of a return line 24, and intake 22 beingconnected to a second control valve 27 via an intake line 26. On theoutput side, first control valve 25 is connected to a return, i.e.,relief line 28 leading to fluid reservoir 21, whereas second controlvalve 27 is connected to output 201 of pressure-supply device 20 on theinput side. Both control valves 25, 27 may be embodied as 2/2 solenoidvalves having spring return. Return line 23 is coupled to twointerconnected discharge openings 231, 232, which are arranged inhousing 15 with axial clearance. Upper discharge opening 231 is coupledto a restrictor 29, and lower discharge opening 232 is disposed in thedisplacement path of operating piston 16 in such a way that operatingpiston 16 is able to close it at a definable distance prior to reachingthe upper limit position. In the exemplary embodiment of FIG. 1, upperdischarge opening 231 is simultaneously used as intake 22, so thatintake line 26 is connected to upper discharge opening 231. Return line24 is connected to lower discharge opening 232, and intake and returnlines 26, 24 are connected to one another via a connecting line 30 inwhich restrictor 29 is disposed.

In the exemplary embodiment of FIG. 4, intake 22 is realized by aseparate intake opening 31 in housing 15. Return line 24, which isconnected to first control valve 25, has two line branches 241, 242, oneof which, line branch 241, leads to upper discharge opening 231 and theother, line branch 242, leads to lower discharge opening 232. Restrictor29, symbolically drawn in in line branch 241, is advantageously realizedby designing upper discharge opening 231 as a throttle bore.

Operating piston 16 is enclosed by a compensating piston 32, which isdisplaceable relative to operating piston 16. Operating piston 16 andcompensating piston 32 are guided in a guide sleeve 33 so as to beaxially displaceable, guide sleeve 33 being fixed in housing 15 in anon-displaceable manner. Compensating piston 32, together with theeffective area of operating piston 16, axially delimits upper pressurechamber 17, and by its annular end face facing away from upper pressurechamber 17 it delimits a compensating chamber 34 in guide sleeve 33.Compensating piston 32 carries a stop 321 near its end facing upperpressure chamber 17, and operating piston 16 carries a counter stop 161on its end forming the effective area, counter stop 161 cooperating withstop 321 in taking along compensating piston 32 by operating piston 16moving into the upper limit position.

As a result of compensating piston 32 and guide sleeve 33, lowerdischarge opening 232 is made up of a first radial bore 35 in housing15, a second radial bore 36 in guide sleeve 33 and a third radial bore37 in compensating piston 32. Compensating chamber 34 is blocked offover the displacement path of operating piston 16; it is released forfluid discharge or fluid intake only at the point where operating piston16, moving into its upper limit position, begins to take alongcompensating piston 32. To this end, a compensation channel 39, whichconnects second radial bore 36 with a radial bore 40 in guide sleeve 33,is worked into guide sleeve 33, radial bore 40 being set apart fromsecond radial bore 36 and discharging toward operating piston 16.Operating piston 16 has an annular groove 41 having an axial groovewidth such that, in a certain relative position of operating piston 16and compensating piston 32, it establishes a connection between themouth of radial bore 40 and compensating chamber 34. To this end,annular groove 41 is placed on operating piston 16 in such a way thatthe connection is established as soon as compensating piston 32 beginsto be carried along by operating piston 16, i.e., with the stop ofcounter stop 161 striking stop 321; the connection is severed again onlywhen operating piston 16 has moved slightly out of its upper limitposition. In the upper limit position of operating piston 16, theconnection between compensating chamber 34 and radial bore 40 ismaintained via annular groove 41, as can be seen in FIG. 3.

Inside upper pressure chamber 17, a spacer sleeve 42, which forms a stopfor compensating piston 32, is inserted in housing 15. Compensatingpiston 32 can thus move between the floor of compensating chamber 34,which is formed by guide sleeve 33, and spacer sleeve 42. Since spacersleeve 42 is located in the region of upper discharge opening 231 andintake opening 31, spacer sleeve 42 is provided with a radial bore 43,as shown in FIG. 1, which corresponds to upper intake opening 231 or todischarge opening 31, which is identical therewith. In the separatedesign of upper discharge opening 231 and intake opening 31 according toFIG. 4, two radial bores 43 are provided, one of which is aligned withupper discharge opening 231 and one with intake opening 31.

The operation of the hydraulic valve actuator is as follows:

In FIG. 1, the valve actuator is shown with operating piston 16 in itslower limit position in which gas-exchange valve 10 is opened to itsmaximum. To close gas-exchange valve 10, control valves 25, 27 areswitched over into their position shown in FIG. 1. First control valve25 is open and upper pressure chamber 17 is thereby connected to fluidreservoir 21 via return line 23 (upper and lower discharge opening 231,232), return line 24 and relief line 28. Second control valve 27 isclosed. Since lower pressure chamber 18 is pressurized at all times bythe fluid pressure generated by pressure-supply device 20, operatingpiston 16 is moved to the right in FIG. 1, and gas-exchange valve 10moves in the closing direction. In the process, fluid is expelled fromupper pressure chamber 17. On one side, the fluid flows off into returnline 24 via lower discharge opening 232 and, on the other side, viaupper discharge opening 231 and restrictor 29, reaching fluid reservoir21 via relief line 28.

In the further course of the closing movement of gas-exchange valve 10,operating piston 16 passes over radial bore 37 in compensating piston32, thereby closing off lower discharge opening 232. Now, the fluid candischarge into return line 24 solely via upper discharge opening 231 andvia restrictor 29. Only a small fluid quantity per time unit is able toflow off through restrictor 29, so that operating piston 16 andgas-exchange valve 10 are decelerated. Operating piston 16 continues adisplacement movement into its upper limit position—now at reducedspeed—until gas-exchange valve 10 is closed, that is to say, until valvemember 12 sets down on valve seat 13.

The displacement stroke in which the deceleration of operating piston 16begins depends on the relative position of operating piston 16 withrespect to compensating piston 32. Compensating piston 32 is able tomove between the base of compensating chamber 34 and spacer sleeve 42.When the internal combustion engine is started up, or during a startingprocedure after the internal combustion engine has been at a standstillfor a longer period of time, compensating piston 32 assumes an arbitraryposition between chamber base and spacer sleeve 42. If compensatingpiston 32 is located too far to the left in the representation in FIG.1, operating piston 16 strikes stop 321 of compensating piston 32 duringvalve closing by way of its counter stop 161. At this moment, annulargroove 41 in operating piston 16 establishes a connection betweencompensating chamber 34, also filled with fluid, and radial bore 40 inguide sleeve 33, which in turn is in connection with lower dischargeopening 232 via compensating channel 39. Compensating piston 32 is nowable to move. Operating piston 16, taking compensating piston 32 along,continues to move until valve member 12 of gas-exchange valve 10 issealingly positioned on valve seat 13. Since compensating piston 32 iscarried along, the connection between compensating chamber 34 and lowerdischarge opening 232 is maintained via annular groove 41 (FIG. 3).

To open gas-exchange valve 10, first control valve 25 is closed andsecond control valve 27 opened. Upper pressure chamber 17 is now underthe fluid pressure supplied by pressure-supply device 20. Since theeffective area of operating piston 16 delimiting upper pressure chamber17 is larger than the effective area of operating piston 16 delimitinglower pressure chamber 18, operating piston 16 moves to the left in thegraphical representation, and gas-exchange valve 10 is opened. Viaannular groove 41, compensating chamber 34 is connected to lowerdischarge opening 232 and the latter is connected to upper pressurechamber 17 via restrictor 29, so that compensating chamber 34 has thesame pressure as upper pressure chamber 17. Since the two effectiveareas of compensating piston 32 that delimit compensating chamber 34 andupper pressure chamber 17 are of the same size, compensating piston 32is pressure-equalized, so that no resulting displacement force isgenerated at compensating piston 32. However, the pressure incompensating chamber 34 is generated somewhat later because ofrestrictor 29, so that compensating piston 32 makes a slight movement tothe left. As soon as operating piston 16 has moved to such an extentthat annular groove 41 breaks off the connection to compensating chamber34, compensating chamber 34 is blocked off, so that compensating piston32 remains in the attained position. In this way, compensating piston 32is aligned, and radial bore 37 in compensating piston 32, which is partof lower discharge opening 232, has a fixed position with respect to theclosed state of gas-exchange valve 10. As a result, operating piston 16always closes radial bore 37 at a fixed distance prior to reaching itslimit position, and the braking operation at gas-exchange vale 10 thusalways begins when valve member 12 is at a fixed distance from valveseat 13. If compensating piston 32 is too far to the right in theclosing operation shown in the representation of FIGS. 1 to 3,compensating piston 32 is adjusted as described during the subsequentclosing and opening operation of gas-exchange valve 10 in thatcompensating piston 32 executes a slight movement to the left.

The valve actuator for a gas-exchange valve 10 shown in, FIGS. 5 and 6conforms to the previously described valve actuator in design andfunctioning method, so that identical components bear matching referencenumerals in this regard. Due to a constructive measure, this valveactuator has the additional advantage that it opens gas-exchange valve10 with high actuating force, so that valve member 12 lifts off fromvalve seat 13 in a rapid and reliable manner, against the high internalpressure in the combustion cylinder of the internal combustion engine,and that it continues to displace valve member 12 with a low actuatingforce once valve member 12 has lifted off from valve seat 13 and theinternal pressure in the combustion cylinder has collapsed as a result.For this purpose, compensating chamber 34, delimited in guide sleeve 33by compensating piston 32, is not connectable to return line 23 viaannular groove 41 in operating piston 16, as shown in FIGS. 1 to 3, butto a fluid reservoir 44, which both accommodates a fluid volume fromcompensating chamber 34 and also fills this fluid volume intocompensating chamber 34. For this purpose, housing 15 and guide sleeve33 are provided with two mutually aligned radial bores 45, 46, which areconnected to a connecting line 47 that leads to fluid reservoir 44. Inthe exemplary embodiment shown, fluid reservoir 44 is designed as aseparate component, but it may also be integrated into housing 15 of thevalve actuator. The connection between compensating chamber 34 and fluidreservoir 44 is established via annular groove 41 again, at the instantwhen compensating piston 32 is carried along by operating piston 16moving into its upper limit position, that is to say, when counter stop161 on operating piston 16 strikes stop 321 on compensating piston 32.

Fluid reservoir 44 has a control chamber 48 provided with two chamberopenings 481, 482 lying axially opposite one another, and a controlmember 49, which is axially displaceable in control chamber 48 for thealternate closing of the two chamber openings 481, 482. Connected to onechamber opening, 481, is connecting line 47 leading to radial bore 45 inhousing 15, whereas the other chamber opening, 482, is connected torelief line 28 via a connecting line. The connection to relief line 28is provided in a line section between the output of first control valve25 and a pressure-modulation valve 51 disposed in relief line 28.Pressure-modulation valve 51 ensures that a slight fluid pressure ofapproximately 0.1 Mpa is always present at chamber opening 481. In theexemplary embodiment of fluid reservoir 44 shown in FIGS. 5 and 6,control member 49 is embodied as a ball, which is able to alternatelyset down on a frustoconical valve seat situated upstream from eachchamber opening 481 and 482, and is thus able to close chamber openings481, 482. Also introduced in control chamber 48 is a radial bore 52,which is connected to connecting line 47 via a throttle 53. Radial bore52 is placed in control chamber 48 in such a way that it lies nearchamber opening 481, but is not covered by control member 49 whencontrol member 49 closes chamber opening 481.

The manner of operation of the valve actuator is as follows:

During closing of the gas-exchange valves, control valves 25, 27 assumethe position shown in FIG. 5, and the closing movement of gas-exchangevalve 10 takes place as described in connection with FIGS. 1 to 3. Inthe process, operating piston 16, which delimits upper pressure chamber17, expels fluid from upper pressure chamber 17 via lower dischargeopening 232 and via upper discharge opening 231 with downstreamrestrictor 29. As soon as control piston 16 passes lower dischargeopening 232, more specifically, radial bore 37 in compensating piston 32associated therewith, the braking operation commences during valveclosing, due to the fact that the fluid now drains solely via restrictor29. With the closing of lower discharge opening 232, counter stop 161 onoperating piston 16 strikes against stop 321 on compensating piston 32,and operating piston 16 takes compensating piston along in its furtherdisplacement travel into upper limit position. Due to the enlargedpiston area (operating piston 16 and compensating piston 32) nowdelimiting upper pressure chamber 17, the braking effect is increased,since, in addition, more fluid must now flow through restrictor 29.Compensating chamber 24 is enlarged by the displacement of compensatingpiston 32, and since annular groove 41 in operating piston 16 hasestablished the connection between control chamber 48 and compensatingchamber 34, fluid is flowing from control chamber 48 into compensatingchamber 34. Via chamber opening 482, fluid flows from relief line 28into control chamber 48, and spherical control member 49 moves to theleft in the illustration until it comes to rest on the valve seatassociated with chamber opening 481 and seals it. If compensating piston32 must still move further to the right in the illustration for thecomplete closing of gas-exchange valve 10, fluid is able to reachcompensating chamber 34 via radial bore 52 and throttle 53. Oncegas-exchange valve 10 is closed completely, operating piston 16 assumesits upper limit position (FIG. 6) in which the connection betweencompensating chamber 34 and control chamber 48 is maintained via annulargroove 41.

To open gas-exchange valve 10, the two control valves 25, 27 areswitched over, so that first control valve 25 closes and second controlvalve 27 opens. Fluid pressure builds up in upper pressure chamber 17,which acts on the effective area of operating piston 16 and on the endface of compensating piston 32. The sum of the effective areas ofoperating piston 16 and compensating piston 32 results in a highdisplacement force in the opening direction of gas-exchange valve 10.Compensating chamber 34 is reduced in size by the displacement movementof compensating piston 32. The fluid is expelled into control chamber48, which causes spherical control member 49 to move to the right incontrol chamber 48. The fluid present in control chamber 48 is expelledinto relief line 28 via chamber opening 482. Via radial bore 52, fluidmay also briefly flow from compensation chamber 32 directly into reliefline 28, but throttle 53 ensures that this is only a very small fluidquantity. With the aid of a non-return valve assigned to throttle 53,this slight flow of fluid may be cut off completely. As soon as controlmember 49 closes other chamber opening 482, no further fluid is able tobe expelled from compensating chamber 34 and compensating piston 32 isunable to execute any further displacement movement. Via the volume incontrol chamber 48, the displacement travel of compensating piston 32may thus be adjusted.

As soon as compensating piston 32 is in a fixed position, operatingpiston 16, which continues to move, lifts off from compensating piston32. The displacement force acting on operating piston 16 issubstantially reduced, since it is only the effective area of operatingpiston 16 delimiting upper pressure chamber 17 that generates thedisplacement force.

Since compensating piston 32 is taken along by operating piston 16during closing of gas-exchange valve 10 until valve member 12 comes torest against valve seat 13, and since compensating piston 32 may travelonly a certain displacement path during opening with the aid of controlchamber 48, it is ensured that lower discharge opening 232, whichcontrols the braking onset during closing of gas-exchange valve 10, isalways in the same position, regardless of thermal expansions andmanufacturing tolerances. As a result, the braking onset does not vary.

Shoulder 322 on compensating piston 32, which can still be seen in FIGS.5 and 6 and which is able to be charged with fluid pressure from intakeline 26 via a connecting line 54 and a radial through-hole 55 throughhousing 15 and guide sleeve 33, is provided for the purpose ofincreasing the wall thickness of compensating piston 32 across a broadregion of compensating piston 32, so as to attain a bettermanufacturability. Theoretically, the outer diameter of compensatingpiston 32 may also be produced without this shoulder 322, if the desiredforce ratio during the initial opening of gas-exchange valve 10 and thesubsequent further opening of gas-exchange valve 10 allows asufficiently large wall thickness of compensating piston 32.

The constructive design of the valve actuator illustrated in FIGS. 5 and6 may be modified in such a way that annular groove 41 in operatingpiston 16 is dispensed with and compensating chamber 34 is permanentlyconnected to control chamber 48. This does not affect the operating modeof the valve actuator. However, it is possible that compensating piston32 is carried along prematurely as a result of friction betweencompensating piston 32 and operating piston 16. However, this can beavoided by observing the manufacturing tolerances.

1-20. (canceled).
 21. An hydraulic valve actuator for activating a gas-exchange valve in a combustion cylinder of an internal combustion engine, comprising: a housing; an operating piston accommodated in the housing, the operating piston being axially displaceable within the housing to an upper limit position which closes the gas-exchange valve and to a lower limit position which maximally opens the gas-exchange valve; an intake line; a return line having upper and lower discharge openings that are mutually connected and are disposed in the housing with axial clearance; lower and upper variable-volume pressure chambers axially delimited by the operating piston, the lower pressure chamber being delimited by a first effective area of the operating piston and being situated so as to be permanently acted on by fluid pressure, the upper pressure chamber being delimited by a second effective area of the operating piston and situated to enable alternate pressurization and depressurization via the intake line and the return line, the first effective area being smaller than the second effective area; and a restrictor coupled to the upper discharge opening; wherein the lower discharge opening is disposed in the displacement path of the operating piston such that the lower discharge opening is closed by the operating piston before the operating piston reaches the upper limit position, and wherein the lower discharge opening is axially displaceable relative to the operating piston.
 22. The hydraulic valve actuator of claim 21, further comprising: a compensating piston enclosing the operating piston and displaceable relative to the operating piston, the compensating piston being configured to be carried along by the operating piston as the operating piston moves into the upper limit position; and a sealable compensating chamber; wherein the lower discharge opening has a first radial bore penetrating the housing, and a second radial bore communicating with the first radial bore is positioned in the compensating piston; and wherein the compensating piston, together with the operating piston, axially delimits the upper pressure chamber in the housing on a first end of the compensating piston, and axially delimits the sealable compensating chamber on a second end of the compensating piston opposite from the first end.
 23. The hydraulic valve actuator of claim 22, wherein the compensating piston includes a stop at the first end facing the upper pressure chamber, and the operating piston includes a counter stop corresponding to the stop, enabling cooperative contact between the operating piston and the compensating piston.
 24. The hydraulic valve actuator of claim 23, wherein the compensating chamber is blocked across the displacement path of the operating piston and is released for a fluid exchange as the operating piston moves into its upper limit position.
 25. The hydraulic valve actuator of claim 24, wherein the operating piston includes an annular groove via which the compensating chamber is connectable to a compensating channel that discharges in the lower discharge opening, an axial width of the annular groove being dimensioned such that the connection between the compensating chamber and the compensating channel is interrupted once the operating piston moves out of the upper limit position.
 26. The hydraulic valve actuator of claim 25, wherein an axial clearance between the annular groove and the counter stop on the operating piston is dimensioned such that, when the stop on the compensating piston and the counter stop on the operating piston come into contact, connection between the compensating chamber and the compensating channel is established via the annular groove.
 27. The hydraulic valve actuator of claim 23, wherein the compensating chamber is configured to be connected to a fluid reservoir once the compensating piston begins to be carried by the operating piston as the operating piston moves into the upper limit position.
 28. The hydraulic valve actuator of claim 27, wherein the compensating chamber is configured to be permanently connected to the fluid reservoir.
 29. The hydraulic valve actuator of claim 27, wherein the operating piston includes an annular groove positioned to establish a connection to the fluid reservoir when contact is made between the stop of the compensating piston and the counter stop of the operating piston.
 30. The hydraulic valve actuator of claim 29, wherein the housing includes a radial bore connected to the fluid reservoir, and an axial groove width of the annular groove is dimensioned such that the annular groove is connectable to an outlet of the radial bore and the compensating chamber.
 31. The hydraulic valve actuator of claim 30, wherein the fluid reservoir includes a control chamber having two chamber openings lying axially opposite one another and a control member axially displaceable in the control chamber, wherein the control member alternately closes one chamber opening and releases the other chamber opening, a first of the two chamber openings being connected to the radial bore in the housing and a second of the two chamber openings being configured to be acted on by a fluid pressure that is slightly greater than a fluid pressure prevailing in the compensating chamber when the operating piston is in the upper limit position.
 32. The hydraulic valve actuator of claim 31, wherein the control chamber is connected to the radial bore in the housing via a throttle.
 33. The hydraulic valve actuator of claim 31, wherein each of the two chamber openings includes a frustoconical valve seat and the control member is configured as a ball.
 34. The hydraulic valve actuator of claim 23, wherein a stop is situated in the upper pressure chamber to delimit displacement of the compensating piston.
 35. The hydraulic valve actuator as recited in claim 34, wherein the stop in the upper pressure chamber includes a spacer ring having a radial bore corresponding to the upper discharge opening in the housing.
 36. The hydraulic valve actuator of claim 35, wherein one of the upper discharge opening and the radial bore is configured as a throttle bore forming a restrictor, and the lower discharge opening and the upper discharge opening are each connected to one of two line branches of the return line.
 37. The hydraulic valve actuator of claim 35, wherein the upper discharge opening simultaneously forms an intake and is connected to the intake line, the lower discharge opening is connected to the return line and the restrictor is disposed in a connecting line coupled to the intake line and to the return line.
 38. The hydraulic valve actuator of claim 36, wherein the return line is configured to be alternatively shut off via a first control valve and connected to a fluid reservoir.
 39. The hydraulic valve actuator of claims claim 31, wherein a pressure-modulating valve having an output is disposed between the first control valve and the fluid reservoir, and the second of the two chamber openings of the control chamber is connected to the output of the pressure-modulating valve.
 40. The hydraulic valve actuator of claim 31, wherein the upper pressure chamber is connected to the intake line configured to be alternatively shut off via a second control valve and connected to a pressure-supply device delivering fluid under high pressure. 